The University of Arizona

Chasing Demons with a Microscope

By Daniel Stolte, University Communications | May 21, 2013

UA physicists glimpse a demon whose existence would violate the laws of physics – but in the strange world of quantum mechanics, they discover that subatomic particles sometimes behave in seemingly demonic ways.

James Clerk Maxwell’s famous thought experiment involves a demon controlling how molecules bounce around in a chamber, seemingly defying the laws of physics. (Image courtesy of Jason Torchinsky http://www.jasontorchinsky.com)
James Clerk Maxwell’s famous thought experiment involves a demon controlling how molecules bounce around in a chamber, seemingly defying the laws of physics. (Image courtesy of Jason Torchinsky http://www.jasontorchinsky.com)
Charles Stafford (left) with the study's first author, former graduate student Justin Bergfield. (Photo: UANews)
Charles Stafford (left) with the study's first author, former graduate student Justin Bergfield. (Photo: UANews)

When you're a scientist working in what is widely considered the most exact of all natural sciences – physics – it is not often you see a demon appear, much less publish a scientific paper about it. Yet that's exactly what a team of physicists in Charles Stafford's group at the University of Arizona did.

While developing complex calculations describing the measurement of temperature across extremely tiny distances such as individual molecules, the group caught a glimpse of "Maxwell's Demon," the hypothetical centerpiece of a famous thought experiment dreamed up by a brilliant 19th-century physicist, but deemed impossible in the real world. 
 
"Maxwell's Demon can't exist because it would violate the laws of thermodynamics," said Charles Stafford, a professor at the department of physics in the UA College of Science. "So you can imagine we were quite surprised to see it appear in our computer-based experiments."
 
Stafford was quick to assure that his group's results are not at odds with the laws of physics, but that its observations have to do with quantum mechanics, where different laws are at play compared to what physicists call the macro world, or anything larger than atoms and molecules. 
 
But who or what is this demon anyway?
 
Scottish physicist James Clerk Maxwell dreamed up a thought experiment in which a box filled with air is set up such that a wall in the middle divides it into two compartments. One is filled with hot air, the other with cold air. Maxwell was one of the first of his time to realize that heat is nothing but the combined motion of particles. One compartment of the box is hot because the air molecules inside bounce around fast and frantically. The other compartment is cold because here, the air molecules drift about slowly and sluggishly. 
 
To understand where the demon comes into play, one has to realize that the fast molecules are fast only on average, just as the slow molecules are slow only on average. Within a given compartment, there will be a whole range of molecules, from super-slow to super-fast and everything in between. It's just that the hot chamber holds more fast molecules, and the cold chamber holds more slow molecules. 
 
Now, what would happen to the temperature in both compartments if the apparatus were simply left to its own devices? Over time, the hot compartment will lose some of its heat to the cold compartment until both have reached equilibrium and share the same temperature. That, in a nutshell, is the second law of thermodynamics, which governs pretty much anything in the universe. 
 
What if, Maxwell mused, there was an invisible creature operating a little trap door in the wall dividing the two chambers? Whenever the demon spots a slow-moving molecule among the fast-moving crowd in the hot compartment, it opens the hatch and lets the molecule cross over into the cold compartment. Likewise, when it spots a fast-moving molecule in the cold compartment where most molecules are slow, it opens the hatch and lets that molecule cross to the other side. 
 
Eventually, all fast moving molecules would end up in the hot chamber, and all the slow ones in the cold chamber. As a result, the temperature difference between the two chambers would be greater than it was in the beginning. It's like water running uphill or a heap of shards spontaneously reassembling into a vase. It simply doesn't happen. Yet, the outcome of this scenario vexed physicists for 120 years, trying to figure out exactly why there couldn't be a Maxwell demon in nature. 
 
"The way one usually gets around this is by analyzing the energy the demon needs to keep track of all the information pertaining to each particle, and the energy necessary to carry out his actions, and showing that when this is included, all is well with the laws of thermodynamics," Stafford explained. 
 
Consequently, Maxwell's Demon has never been observed in reality. But, as Stafford's research team discovered, one only has to leave the familiar world around us and enter the realm of quantum mechanics, and the demon might just pop up in the data. 
 
"We show through our extensive numerical simulations that if you try to measure the temperature of a system of particles, in this case, electrons, not molecules in a box, with a spatial precision down to the size of individual atoms, then the laws of quantum mechanics result in an effect that is almost identical to what Maxwell's demon would do," Stafford explained. 
 
In their numerical experiments, the group simulated a system – Maxwell's box, if you will – consisting of a small molecule of carbon and hydrogen atoms with three electrodes attached to it. One electrode transfers heat into the molecule, the second electrode drains heat out of the molecule, and the third measures the temperature at different places within the molecule. The whole setup is called scanning thermal microscopy: A scanning electron microscope uses an ultrafine tip whose apex consists of a single atom to measure temperatures on an atomic scale. 
 
The idea behind this setup, although not directly applicable at this early stage, could one day be used in devices that turn heat into electrical power or vice versa. 
 
"In our simulations, we found that it is possible to separate the hot from the cold electrons within that single molecule without expending any energy to make this happen, which is exactly what Maxwell's Demon does," Stafford said. 
 
However, it turns out this sorting process does not violate the laws of thermodynamics because of the peculiarities of quantum physics, he explained. 
 
"In the quantum state of the molecule, the hot and cold electrons never mix despite the fact that they exist in the same place at the same time. But that's because they 'remember' where they came from due to quantum wave effects – not because there is a demon at play," he said.
 
The research project and its unexpected results were several years in the making, Stafford said. The investigation began when undergraduate researcher Shauna Story, who graduated with a Bachelor of Science in physics in 2010, discovered the strange effect while studying simple molecules. This led the group to tests with more complex structures, resulting in a publication co-authored by former graduate student Justin Bergfield and organic chemist Robert Stafford. 
 

Contacts

Researcher Contact:

Charles Stafford

Department of Physics

520-626-4260

stafford@physics.arizona.edu

 

Media Contact:

Daniel Stolte

University Communications

520-626-4402

stolte@email.arizona.edu